1. Field
Embodiments of the present invention relate to the characterization of non-linear optical materials. More specifically, embodiments of the present invention relate to using Bragg coupling to determine optical properties of waveguide regions of non-linear optical materials.
2. Technical Background
Non-linear optical materials such as non-linear optical crystals may be utilized in optical systems to generate higher harmonic waves of a fundamental laser signal. For example and by way of illustration, not limitation, short wavelength sources may be configured for high-speed modulation by combining a single-wavelength semiconductor laser, such as a distributed feedback (DFB) laser, a distributed Bragg reflector (DBR) laser, a vertical cavity surface-emitting laser (VCSEL), a vertical external cavity surface-emitting laser (VECSEL), or a Fabry-Perot laser, for example, with a light wavelength conversion device, such as a second harmonic generation (SHG) crystal or a higher harmonic generating crystal. SHG crystals use second harmonic generation properties of non-linear crystals to frequency-double laser radiation. For example, a SHG crystal may be configured to generate green light by converting the wavelength of a 1060 nm DBR or DFB laser to 530 nm.
In many applications, such as laser projection systems, optical properties of the wavelength conversion device are critical to system performance. Particular optical properties may include propagation loss, peak conversion wavelength and optical power conversion efficiency, among others. For example, the conversion efficiency of a SHG crystal, such as MgO-doped periodically poled lithium niobate (PPLN), is strongly dependent on the wavelength matching between the laser diode and the SHG device. The bandwidth of a PPLN SHG device is often very small—for a typical PPLN SHG wavelength conversion device, the full-width half-maximum (FWHM) wavelength conversion bandwidth is only in the 0.16 to 0.2 nm range and mostly depends on the length of the crystal. Once the semiconductor laser wavelength deviates outside the wavelength conversion bandwidth of the PPLN SHG device, the output power of the conversion device at the target wavelength drops.
Wavelength conversion devices such as PPLN SHG devices are often fabricated from a wafer of a non-linear material such as lithium niobate or lithium tantalate that may contain a plurality of wavelength conversion devices defined by a plurality of waveguide regions to be diced from the wafer. The wafer comprises a waveguide layer of periodically poled non-linear material that is adhered to a substrate. The waveguide layer may be periodically poled by applying a voltage to a pattern that is applied to the wafer via photolithography, for example. The conversion center wavelength is determined by the period of the poling as well as by the details of the waveguide geometry and index distribution. The tolerance requirements may be very demanding. Some waveguides may fall outside of the acceptable tolerance such that further processing of these waveguides is undesirable.
Although wavelength conversion devices must meet strict tolerance requirements, current mass production methods do not provide for the testing or characterization of wavelength conversion devices during the fabrication process. As such, the devices are commonly tested after fabrication is completed. For example, testing methodologies require injecting light into and extracting it from the endfaces of the wavelength conversion devices after the dicing and endface polishing of the individual wavelength conversion devices from the wafer. Further, current methods do not allow the many waveguide regions of the wafer to be tested concurrently.
Testing SHG devices and rejecting failures after fabrication results in wasted resources and production time. If defects in a portion or portions of the wafer could be detected early in the fabrication process, the defective portions could be discarded. Further processing may then be limited to those portions that meet the optical properties or requirements, thereby decreasing processing costs and increasing production yield.